Portable communication devices, WLAN routers, etc. or, more generally: transmission/reception apparatuses which communicate by means of RF signals, require RF filters in order to separate desired from undesired signals. By way of example, such filters can be interconnected in front-end circuits, e.g., in duplexers.
Here, the filter should assume the object of distributing the signals between a chipset and optionally additionally present filters. The circuit outlay should be as little as possible. The filter should be compatible with a multiplicity of different filter technologies of further filters, enable a small installation size of a corresponding device and, in particular, permit a high selectivity.
The trend toward communication devices which can handle ever more frequency bands leads to complicated interconnections of various filters for the different frequency bands. Therefore, the need arises for tunable filters in order to be able to use different frequency bands with the same filter.
Previous solutions to these requirements are substantially based on extending known filter circuits by tunable impedance elements, or based on the use of switches, by means of which filter elements can be switched into a filter topology.
The contribution “Tunable Filters Using Wideband Elastic Resonators”, Kadota et al., IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 60, no. 10, October 2013, pages 2129-2136, has disclosed filter circuits, in which tunable capacitors are added to RF filters with acoustic resonators.
The contribution “A Novel Tunable Filter Enabling Both Center Frequency and Bandwidth Tunability”, Inoue et al., Proceedings Of The 42nd European Microwave Conference, 29 Oct.-1 Nov. 2012, Amsterdam, The Netherlands, pages 269-272, has disclosed RF filters with tunable capacitors and tunable inductances.
The contribution “RFMEMS-Based Tunable Filters”, Brank et al., 2001, John Wiley & Sons, Inc. Int J RF and Microwave CAE11: pages 276-284, 2001, has also disclosed interconnections of L and C elements, with the capacitances of the capacitive elements being adjustable.
The contribution “Tunable Isolator Using Variable Capacitor for Multi-band System”, Wada et al., 978-1-4673-2141-/13/S31.00, 2013 IEEE MTT-S Symposium or the publication document WO2012/020613 has disclosed the use of insulators in RF filters. In general, tunable filters require a multiplicity of components, which is greater than that of conventional known filters tuned to one frequency band. Moreover, there is more complicated interconnection which causes further problems as conductor track crossings and the couplings connected therewith are practically unavoidable.
The multiplicity of additionally required components for tunable filters moreover leads to more space being required.